FUNCTION" OF TIIK KXTKKXAL AM) M1DDLIO EARS (W.'J 



its neighborhood, while it is quite unaffected by others. In other words, it 

 can take up and vibrate in response only to those tones whose vibrations 

 nearly correspond in number with those of its own fundamental tone. The 

 tympanic membrane can take up an immense range of tones produced by 

 vibrations ranging from 30 to 4,000 or 5,000 per second. This would be clearly, 

 impossible if it were an evenly stretched membrane. The fact is that the 

 membrana tympani is by no means evenly stretched, and this is due partly 

 to its slightly funnel-like form, and partly to its being connected with the 

 chain of auditory ossicles. Further, if the membrane were quite free in its 

 center, it would go on vibrating as a drum does some time after it is struck; 

 each sound would be prolonged, leading to considerable confusion. This 



B 



fie. 439- Showing A and B, Simple Pendular Vibrations, Separated by One Octave. C, 

 The form of the curve produced by the combination of A and B. 



evil is obviated by the ear bones, which check the continuance of the vibra- 

 tions like the "dampers" in a piano. 



The vibrations of the membrana tympani are transmitted by the chain 

 of ossicles to the fenestra ovalis and fluid of the labyrinth, their dispersion 

 in the tympanum being prevented by the difficulty of the transition of vibra- 

 tions from solid to gaseous bodies. The necessity of the presence of air on 

 the inner side of the membrana tympani, in order to enable it and the auditory 

 ossicles to fulfil the objects just described, is obvious. Without this pro- 

 vision, neither would the vibrations of the membrane be free nor the chain 

 of bones isolated, so as to propagate the sonorous undulations with con- 

 centration of their intensity. But while the oscillations of the membrana 

 tympani are readily communicated to the air in the cavity of the tympanum, 

 those of the solid ossicles will not be conducted away by the air, but will 

 be propagated to the labyrinth without being dispersed in the tympanum. 



The propagation of sound through the auditory ossicles to the labyrinth 

 must be effected by oscillations of the bones as a whole. 



The existence of the membrane over the fenestra rotunda permits approxi- 



